Harvesters such as windrowers, tractors, and forage harvesters, have to operate effectively in normal and high-speed modes. Typical construction for such vehicles include front ground wheels mounted on the frame at fixed angles parallel to each other and parallel to a center line of the frame and rear ground wheels mounted on a respective caster. Each of the front ground wheels is typically driven by a respective drive motor which allows variable speed in both the forward and reverse directions such that steering of the tractor is effected by a differential in speed between the front wheels with the rear wheels following the steering in a castering action.
The front wheels only are driven and are driven in a manner which allows full control in the forward and reverse directions so that steering is effected by a differential speed between the two front wheels with the rear wheels following in the conventional castering action. The hydraulic differential or dual path steering of traditional harvesters is generally unstable during high-speed operation. The steering generally functions by varying the speed of the right and left-hand side drive wheels by adjusting the flow to the right-hand and left-hand ground drive motors through the mechanical varying of the swash-plate angles of the ground drive pumps. The steering of the vehicle results when the inside wheels slows and the outside wheel speeds up. Such systems generally require that the ground drive motors are able to overcome the momentum of the harvester when initiating a turn, which can force the inside wheel in a coasting operation that must be overcome or dissipated before the harvester can turn.
Overcoming or dissipating the coasting operation of the inside wheel results in delayed steering reaction, which can negatively affect the overall steering performance of the harvester. The addition of rear steer can assist in overcoming such delay. However, such systems retain front differential steering during high-speed operation, which due to variances in wheel motor efficiencies, steering cylinder calibrations, or the like, can cause the front differential steering to be out of phase with the rear steer. This can result in the front steering fighting or resisting the rear steer, causing the harvester to operating in a darting or unstable manner during high-speed road operation. Thus, conventional front wheel steering systems for harvesters having rear steer operate in an unstable manner during high-speed road operation.
For example, FIG. 1 shows a perspective view of a conventional windrower 10 having rear steer. The windrower 10 generally includes front wheels 12, 14 rotatably mounted to a frame 16, and a rear suspension system 18 mounted to the frame 16. The windrower 10 includes a cabin 20 configured and dimensioned to receive an operator, and having a plurality of controls for operation of the windrower 10, such as controlling a header (not shown) attachable to the front 22 of the windrower 10, controlling movement of the windrower in a forward direction 24, and controlling movement of the windrower in a reverse direction 26. At the rear 28, the windrower 10 includes casters 30, 32 rotatably mounted on opposing sides of the frame 16, each caster 30, 32 including a caster wheel 34, 36.
FIG. 2 shows a diagrammatic view of a conventional steering system 38 that can be mounted to the frame 16 of the windrower 10. The conventional steering system 38 is of the type disclosed in U.S. Pat. No. 8,997,902, the disclosure of which is incorporated herein by reference in its entireties. The steering system 38 generally includes a steering wheel 40 connected to a steering control valve 42. The steering control valve 42 is connected to a steering drive motor 44 via hydraulic hoses or lines 46, 48. The steering system 38 includes a pump 50 and a reservoir 52 connected to the steering control valve 42 via hydraulic hoses or lines 54, 56, respectively. The pump 50 can be in communication with an engine 58 which is mounted to the frame 16.
The steering system 38 includes drive pumps 60, 62 connected to a control input steering assembly 64 by pintel arms 66, 68. The steering system 38 includes an output shaft 70 with a coupling 72 for connection with the control input shaft 64. The steering system 38 includes a speed selector 74 transmitting an output signal 76 to a hydraulic valve 78. The hydraulic valve 78 is connected to an actuator 80, the actuator 80 being connected to a neutral bias element 82. The actuator can be mounted to a bracket 81.
FIG. 3 is a hydraulic circuit of the conventional steering system 38. The hydraulic circuit shows the steering wheel 40 with the steering control valve 42. The hydraulic circuit shows the right-hand ground drive motor 84 connected with the right-hand ground drive pump 62, and the left-hand ground drive motor 86 connected with the left-hand ground drive pump 60. The pumps 60, 62 are connected to a steering screw 88 in communication with a steering motor 44. As noted above, the front wheels only are driven in a manner which allows full control in the forward and reverse directions so that steering is effected by a differential speed between the two front wheels with the rear wheels following in the conventional castering action. The hydraulic differential or dual path steering of the steering system 38 is generally unstable during high-speed operation.